An enthralled White House crowd of more than 13,500 sang "Happy Birthday" to Pope Benedict XVI today - twice - as he celebrated his 81 birthday with President George Bush.

Guests packed the sun-drenched South Lawn as Benedict became only the second pope ever to visit the White House and the first in nearly 30 years.

He received a 21-gun salute and heard Kathleen Battle, the American soprano, produce a haunting rendition of The Lord's Prayer .

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Hello there: The pontiff passes in front of the White House in his popemobile

Historic: Pope Benedict with President George W Bush and his wife Laura on a White House balcony

President Bush said that the visit was a reminder for Americans to "distinguish between simple right and wrong".

"We need your message to reject this dictatorship of relativism and embrace a culture of justice and truth," Mr Bush added.

"In a world where some see freedom as simply the right to do as they wish, we need your message that true liberty requires us to live our freedom not just for ourselves, but in a spirit of mutual support."

The pontiff's 90-minute stay at the White House was accompanied by the kind of pomp and pageantry rarely seen even on grounds accustomed to routinely welcoming royalty and the world's most important leaders.

Lampposts fluttered with flags in the red-white-and-blue of America and yellow-and-white of the Holy See.

The vast South Lawn was filled to nearly bursting with the largest crowd of Bush's presidency, requiring a large television screen so those farther back could see.

Thousands unable to get inside filled Washington's streets as well, playing music and waving banners as they waited for a hoped-for glimpse of the pontiff passing by later in his popemobile.

An almost serenely quiet papal arrival at the White House preceded the program as Benedict's limousine pulled up to a greeting from Bush and his wife, Laura.

The two leaders strolled along a red carpet to a platform set up on the lawn, and sat side-by-side as the Marine Band played the national anthem of the Holy See while a 21-gun salute sprayed gray smoke into the air.

Celebration: Pope Benedict greets the crowd on the White House lawn after they sand Happy Birthday to him

he love in the audience was evident. A few yelled "Viva il Papa."

Four toddlers sat on the grass with handmade signs, one reading "We love you pope of hope" and the other showing a birthday cake - chocolate - with an 81 on it.

"Happy Birthday" was sung spontaneously at first early in the ceremony, and a formal, more full-throated version came at the end.

"God bless America," said Benedict robustly, to cheers from the excited throng.

The pontiff has said he was looking forward to meeting a "great people and a great church" during his first papal journey to the United States.

The six-day trip to Washington and New York City coincides not just with his birthday, but the three-year anniversary of his ascendancy to the Roman Catholic Church's top position.

Nurturing the U.S. flock is a sensitive and important mission for Benedict at a time, not just of ongoing scandal in the American church but amid his campaign to tamp down secularism and re-ignite faith worldwide.

"I trust that my presence will be a source of renewal and hope for the church in the United States and strengthens the resolve of Catholics to contribute even more responsibly to the life of this nation of which they are proud to be citizens," Benedict said at Bush's side.

Birthday surprise: More than 13,000 people packed on to the White House's south lawn to see Pope Benedict

Bush showed off America to its important visitor, ticking off what he said are its best virtues: a nation of prayer and compassion and one that is the most "innovative, creative and dynamic country on Earth" but also among the most religious.

"Most of all, Holy Father, you will find in America people whose hearts are open to your message of hope," Bush said. But while acting the proud father, Bush also seemed to suggest that America could use a little tough talking-to by the pontiff.

"In a world where some treat life as something to be debased and discarded, we need your message that all human life is sacred and that each of us is willed, each of us is loved, and each of us is necessary," the president said, drawing sustained applause.

Adela Arguello, a Department of Homeland Security worker from Miami was touched. "We're living in very terrible times and any message like this is important," she said. "He needed to come."

"How often in life do you get to sing Happy Birthday to the pope?" said Brenda Hawk, a Sunday school teacher. "Even if you're not Catholic, it's darn cool!"

The president kicked off the unprecedented series of papal festivities on Tuesday, by motoring to Andrews Air Force Base outside Washington to meet Benedict's plane, something he has never done for any leader.

While the pontiff received a rock-star reception from hundreds of Catholic students and others, Bush stood back in the unusual role of second fiddle.

On Wednesday, Bush and his wife, Laura, escorted the pontiff from the South Lawn ceremony to the White House's Blue Room to introduce him to relatives and serve birthday cake.

The leaders then strolled down the colonnade for about 45 minutes of talks in the Oval Office, the 25th meeting between a Roman Catholic pope and a U.S. president - sessions that span 89 years, five pontiffs and 11 American leaders.

A long motorcade accompanying the pontiff then exited the White House grounds for a slow procession along Pennsylvania Avenue. As Benedict waved from a chair inside his glass-walled, white Mercedes popemobile, spectators responded with loud cheers and applause.

Across from the White House, though, the papal visit drew a protest of about 200. With one demonstrator dressed as the pope, the crowd held signs criticizing celibacy and a large banner reading "100,000 sexually abused kids in the US."

In the evening, the Bushes are hosting a dinner in Benedict's honor, complete with Bavarian-style food to celebrate his native Germany. But a meeting between Benedict and U.S. bishops at the Basilica of the National Shrine of the Immaculate Conception in Washington was preventing him from dining with the president.

These two leaders share much common ground, particularly in opposing abortion, gay marriage and embryonic stem cell research. But there are plenty of differences.

They disagree over the war in Iraq, the death penalty and the U.S. trade embargo against Cuba. Benedict also speaks for environmental protection and social welfare in ways that often run counter to Bush policies.

And the pontiff told reporters on his plane that he planned to bring up immigration policy with Bush during their private Oval Office meeting. Benedict has talked forcefully in the past about the damage caused by punitive immigration laws.

A joint U.S.-Holy See statement issued afterward suggested that Benedict followed through. It said the leaders discussed "the need for a coordinated policy regarding immigration, especially their humane treatment and the well being of their families."

On Iraq, the statement said discussion focused particularly on the fears of the Christian minority in the Muslim-majority country.

Other topics included human rights, religious freedom, fighting poverty and disease in Africa, the Israeli-Palestinian dispute, Lebanon and terrorism.